1. Field of the Invention
The invention relates generally to methods for creating refractive index changes in substrates and waveguides, and more particularly, to methods for creating refractive index changes by up-conversion of rare earth ions.
2. Technical Background
Creating refractive index changes in substrate materials, such as glass, is a very useful process that may be used to create waveguide structures. In addition to creating the waveguide, it is also useful to modify the refractive index profile of an existing waveguide (e.g. optical fiber) to make grating structures. Alternatively, modifying the refractive index profile may be used as an optical storage medium, as used in producing holographic images for example.
Presently, refractive index changes in optical fiber are created by a number of conventional processes. One such technique utilizes two photon absorption of blue (e.g. 488 nm) light from an Argon ion laser. The blue photons are absorbed by the glass. The absorption of the photons by the glass generates refractive index changes in the glass. The manufacture of very long gratings (e.g. meters in length) may be well suited by this technique since the exposure occurs simultaneously along the whole fiber length.
This technique, however, is relatively expensive. High power blue lasers are required to cause a sufficient amount of photon absorption to obtain the desired change in the refractive index to occur. Such lasers are generally very expensive.
Even when these expensive blue lasers are used, they are best suited to only certain types of optical fiber. Although silica fiber is transparent to blue light, standard fiber is highly multi-moded at blue photon wavelengths having a cutoff wavelength in the IR. However, most practical applications would require the fiber to be single moded at the writing wavelength, hence requiring special fiber for this application.
Another technique utilizes ultra violet (UV) light that is absorbed by the glass creating defects in the glass, which generates the desired refractive index changes. The UV light is commonly in the range of about 248 nm to about 193 nm. A dopant is often incorporated into a portion of the glass material (typically core material) to strongly increase the UV absorption to create refractive index changes. Common silica glass dopants include germanium, boron, tin, and cerium. For these dopants, absorption will generally be very strong, for example in the range of 0.1 dB to about 100 dB per micron.
The strong absorption results in limited penetration depth of the UV light. Consequently, conventional UV manufacturing techniques cannot propagate UV light along the optical fiber, as the penetration depth is not deep enough to propagate along the whole length of the optical fiber. Instead, conventional UV manufacturing techniques expose the side of the optical fiber, one section at a time. Side exposure is limited by the phase mask size which is usually less than 10 cm at best, and the UV beam diameter which determines penetration depth of the UV light. Thus conventional UV manufacturing techniques are only capable of creating refractive index changes in sections of less than 10 cm in length. To create a refractive index change in substrates longer than 10 cm, the entire setup must be moved to the corresponding next 10 cm section, realigned, and run again. As exposure to the UV light may take several hours to create the refractive index changes, long lengths of substrate cannot be manufactured quickly using a conventional UV light process. Further, moving the entire setup requires extremely careful setup to align the refractive index changes of the corresponding sections, or else the refractive index change will not be uniform throughout the optical fiber, and will not have the tolerances required for optical signal transmission.
Creating waveguide structures in bulk substrate materials can be achieved by a number of different mechanisms. Using light to directly create the refractive index change is either done by direct absorption of UV light into the bandgap of the material, which again suffers from strong absorption and hence limited penetration depth, or very high order multiphoton absorption typically using high intensity short pulse (100 fsec) lasers. Very high order multiphoton absorption suffers from the expense and complication of the required laser sources, as well as the difficult control over the non-linear propagation of ultra short pulses through the substrate material, often resulting in non-optimized waveguides.
One aspect of the present invention includes providing a method for creating refractive index changes in a substrate by irradiating the substrate with infrared (IR) radiation or visible light. Ultra-violet (UV) radiation is generated in the substrate responsive to the IR radiation such that the change in the refractive index of the substrate is generated responsive to the UV radiation. Preferably, the substrate includes a glass doped with rare earth ions. The rare earth ions may include at least Tm3+ ions.
Another aspect of the present invention is directed to a substrate including a substrate matrix material, at least one type of rare earth ion dopant in the matrix material, and at least one defect in the matrix material which affects a refractive index of the substrate, and which was created by ultra-violet (UV) radiation emission from the rare earth ion dopants responsive to infrared (IR) radiation. Preferably, the UV radiation is created by an up-conversion process that converts IR radiation or visible radiation to UV radiation. The substrate may include an optical fiber, whose core is doped with Tm3+ ions.
Another aspect of the present invention includes a method of making a glass substrate including the steps of doping the glass substrate with fluorescent ions, and pumping infrared (IR) radiation or visible light radiation into the glass substrate. Ultra-violet (UV) radiation is generated in the substrate by the fluorescent ions responsive to the IR or visible radiation to create defects in the substrate such that a change in a refractive index is generated. Preferably, the step of pumping IR or visible radiation is performed by a semiconductor pump laser.